Golf ball

ABSTRACT

Golf ball  2  has a spherical core  4,  a mid layer  6  covering this core  4,  a reinforcing layer  8  covering this mid layer  6 , and a cover  10  covering this reinforcing layer  8 . Base polymer of this mid layer  6  includes an ionomer resin as a principal component. Base polymer of the cover  10  includes a thermoplastic polyurethane elastomer as a principal component. The cover  10  has a weight Wc of equal to or less than 3.2 g. The cover  10  has a hardness Hc as measured with a Shore D type hardness scale of equal to or less than 55. A ratio (Hc/Wc) of the hardness Hc to the weight Wc (g) is equal to or greater than 10.0. The reinforcing layer  8  includes a thermosetting resin as a base polymer. The reinforcing layer  8  has a thickness of 3 μm or greater and 50 μm or less. The reinforcing layer  8  has a tensile strength of 150 kgf/cm 2  or greater and 500 kgf/cm 2  or less.

This application claims priority on Patent Application No. 2004-221049filed in JAPAN on Jul. 29, 2004, the entire contents of which are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to golf balls. More particularly, thepresent invention relates to multi piece golf balls having a core, a midlayer and a cover.

2. Description of the Related Art

Top concern to golf players for golf balls is their flight performances.Golf balls that are excellent in flight performances are responsible forscores. The golf players particularly place great importance on flightdistance attained upon shots with a driver.

Golf players also place great importance on spin performances of golfballs. Great back spin rate results in small run. For golf players, golfballs which are liable to be spun backwards are apt to be rendered tostop at a targeted position. Great side spin rate results in easilycurved trajectory of the golf ball. For golf players, golf balls whichare liable to be spun sidewise are apt to allow their trajectory tocurve intentionally. Golf balls that are excellent in spin performancesare excellent in control performances. High-class golf playersparticularly place great importance on control performances upon shotswith a short iron.

In light of the flight performances and control performances, golf ballshaving a variety of structures have been proposed. For example, U.S.Pat. No. 6,106,415 discloses a golf ball having a core, a mid layercomprising an ionomer resin and a cover comprising polyurethane.

Covers having high elasticity are advantageous in terms of the flightperformance. However, highly elastic covers liable to deteriorate thecontrol performance. Soft covers are advantageous in terms of thecontrol performance. However, the soft covers are disadvantageous interms of the flight performance. Demands from the golf players for golfballs have increasingly escalated in recent years. Highly balancedflight performances and control performances have been desired. Anobject of the present invention is to provide golf balls that areexcellent in flight performances and control performances.

SUMMARY OF THE INVENTION

A golf ball according to the present invention has a spherical core, amid layer positioned outside of this core, and a cover positionedoutside of this mid layer. Principal component of the base polymer ofthis mid layer is an ionomer resin. This mid layer has a hardness Hm asmeasured with a Shore D type hardness scale of equal to or greater than62. This cover has a hardness Hc as measured with a Shore D typehardness scale of equal to or less than 55. This cover has a weight Wcof equal to or less than 3.2 g. Ratio (Hc/Wc) of the hardness Hc to theweight Wc (g) of this cover is equal to or greater than 10.0.

Because low head speed is achieved upon shots with a short iron, amountof deformation of the golf ball is small. Upon shots with a short iron,spin rate primarily depends on the material of the surface of the cover.Because the golf ball according to the present invention has a coverhaving a hardness Hc of equal to or less than 55, slipping that occursbetween the club face and the golf ball upon impact is suppressed.According to this golf ball, a great spin rate is achieved upon a shotwith a short iron. This golf ball is excellent in a control performanceupon a shot with a short iron.

Upon shots with a driver, the mid layer and the core are also deformedgreatly in addition to the cover. Covers having a low hardness Hc may bedisadvantageous in terms of resilience performances. Less adverseeffects from the cover are exerted on the resilience performance becausethe cover has a weight Wc of equal to or less than 3.2 g and the ratio(Hc/Wc) of equal to or greater than 10.0 in this golf ball. Greatresilience performance is accomplished by providing the mid layer inwhich an ionomer resin is used, and which has a hardness Hm of equal toor greater than 62. This golf ball is excellent in a flight performanceupon a shot with a driver.

Preferably, principal component of the base polymer of the cover is athermoplastic polyurethane elastomer. Preferably, the cover has athickness Tc of equal to or less than 0.6 mm.

Preferably, a reinforcing layer is provided which comprises athermosetting resin as a base polymer between the mid layer and thecover. Preferably, this reinforcing layer has a thickness of 3 μm orgreater and 50 μm or less. Preferably, this reinforcing layer has atensile strength of 150 kgf/cm² or greater and 500 kgf/cm² or less.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cut off cross-sectional view illustrating a golfball according to one embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is hereinafter described in detail withappropriate references to the accompanying drawing according to thepreferred embodiments.

A golf ball 2 depicted in FIG. 1 has a spherical core 4, a mid layer 6covering this core 4, a reinforcing layer 8 covering this mid layer 6,and a cover 10 covering this reinforcing layer 8. Numerous dimples 12are formed on the surface of the cover 10. Of the surface of the cover10, a part except for the dimples 12 is a land 14. Although this golfball 2 has a paint layer and a mark layer to the external side of thecover 10, these layers are not shown in the Figure.

This golf ball 2 has a diameter of from 40 mm to 45 mm. From thestandpoint of conformity to a rule defined by United States GolfAssociation (USGA), the diameter is preferably equal to or greater than42.67 mm. In light of suppression of the air resistance, the diameter ispreferably equal to or less than 44 mm, and more preferably equal to orless than 42.80 mm. This golf ball 2 has a weight of 40 g or greater and50 g or less. In light of attainment of great inertia, the weight ispreferably equal to or greater than 44 g, and more preferably equal toor greater than 45.00 g. From the standpoint of conformity to a ruledefined by USGA, the weight is preferably equal to or less than 45.93 g.

The core 4 is usually obtained through crosslinking of a rubbercomposition. Examples of preferred base rubber include polybutadienes,polyisoprenes, styrene-butadiene copolymers, ethylene-propylene-dienecopolymers and natural rubbers. In light of the resilience performance,polybutadienes are preferred. When other rubber is used in combinationwith polybutadiene, it is preferred that polybutadiene is included as aprincipal component. Specifically, the proportion of polybutadieneoccupying in total base rubber is preferably equal to or greater than50% by weight, and particularly preferably equal to or greater than 80%by weight. Polybutadienes having a percentage of cis-1,4 bond of equalto or greater than 40%, and particularly equal to or greater than 80%,are preferred.

For crosslinking of the core 4, a co-crosslinking agent is usually used.Preferable co-crosslinking agent in light of the resilience performanceis a monovalent or bivalent metal salt of an α,β-unsaturated carboxylicacid having 2 to 8 carbon atoms. Specific examples of preferableco-crosslinking agent include zinc acrylate, magnesium acrylate, zincmethacrylate and magnesium methacrylate. Zinc acrylate and zincmethacrylate are particularly preferred on the ground that a highresilience performance can be accomplished.

Also, an α,β-unsaturated carboxylic acid having 2 to 8 carbon atoms, andan oxidized metal may be blended as a co-crosslinking agent. Bothcomponents react in the rubber composition to give a salt. This salt isresponsible for the crosslinking reaction. Examples of preferableα,β-unsaturated carboxylic acid include acrylic acid and methacrylicacid. Examples of preferable oxidized metal include zinc oxide andmagnesium oxide.

The amount of the co-crosslinking agent to be blended is preferably 10parts by weight or greater and 50 parts by weight or less per 100 partsby weight of the base rubber. When the amount is less than the aboverange, the resilience performance of the golf ball 2 may becomeinsufficient. In this respect, the amount is more preferably equal to orgreater than 15 parts by weight. When the amount is beyond the aboverange, hard feel at impact of the golf ball 2 may be experienced. Inthis respect, the amount is more preferably equal to or less than 45parts by weight.

It is preferred that an organic peroxide is blended together with theco-crosslinking agent into the rubber composition for use in the core 4.The organic peroxide serves as a crosslinking initiator. By blending theorganic peroxide, the resilience performance of the golf ball 2 may beimproved. Examples of suitable organic peroxide include dicumylperoxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane,2,5-dimethyl-2,5-di(t-butylperoxy)hexane and di-t-butyl peroxide.Particularly versatile organic peroxide is dicumyl peroxide.

The amount of the organic peroxide to be blended is preferably 0.1 partby weight or greater and 3.0 parts by weight or less per 100 parts byweight of the base rubber. When the amount is less than the above range,the resilience performance of the golf ball 2 may become insufficient.In this respect, the amount is more preferably equal to or greater than0.3 part by weigh, and particularly preferably equal to or greater than0.5 part by weight. When the amount is beyond the above range, hard feelat impact of the golf ball 2 may be experienced. In this respect, theamount is more preferably equal to or less than 2.5 parts by weight.

A filler may be blended into the core 4 for the purpose of adjustingspecific gravity and the like. Illustrative examples of suitable fillerinclude zinc oxide, barium sulfate, calcium carbonate and magnesiumcarbonate. Also, powder consisting of a highly dense metal may beblended as a filler. Specific examples of the highly dense metal includetungsten and molybdenum. The amount of the filler to be blended isdetermined ad libitum so that the intended specific gravity of the core4 can be accomplished. Particularly preferable filler is zinc oxide.Zinc oxide serves not only in adjusting specific gravity but also as acrosslinking activator. Various kinds of additives such as sulfur, asulfur compound, an anti-aging agent, a coloring agent, a plasticizer, adispersant and the like may be blended in an appropriate amount to thecore 4 as needed. The core 4 may be also blended with crosslinked rubberpowder or synthetic resin powder.

Amount of compressive deformation of the core 4 is preferably equal toor less than 5.0 mm, more preferably equal to or less than 4.5 mm, andparticularly preferably equal to or less than 4.0 mm. When the golf ball2 is hit with a driver, the core 4 is also deformed greatly inconjunction with the cover 10 and the mid layer 6. The core 4 having asmall amount of compressive deformation is responsible for a flightperformance upon a shot with a driver. Too small amount of compressivedeformation may result in deteriorated feel at impact. In light of thefeel at impact, the amount of compressive deformation is preferablyequal to or greater than 1.5 mm, and particularly preferably equal to orgreater than 2.0 mm.

For the measurement of the amount of compressive deformation, a core 4is first placed on a hard plate made of metal. Next, a cylinder made ofmetal gradually descends toward the core 4. Accordingly, the core 4,which is sandwiched between the bottom face of the cylinder and the hardplate, is deformed. A migration distance of the cylinder, starting fromthe state in which initial load of 98 N is applied to the core 4 up tothe state in which final load of 1274 N is applied thereto is the amountof compressive deformation.

The core 4 preferably has a diameter of 25 mm or greater and 41.5 mm orless. The core 4 preferably has a weight of 25 g or greater and 42 g orless. Crosslinking temperature of the core 4 is usually 140° C. orgreater and 180° C. or less. The crosslinking time period of the core 4is usually 10 minutes or longer and 60 minutes or less. The core 4 maybe formed with two or more layers.

A thermoplastic resin composition is suitably used for the mid layer 6.Examples of the base polymer of the resin composition include ionomerresins, thermoplastic polyester elastomers, thermoplastic polyamideelastomers, thermoplastic polyurethane elastomers, thermoplasticpolyolefin elastomers and thermoplastic polystyrene elastomers. Inparticular, ionomer resins are preferred. Ionomer resins are highlyelastic. The ionomer resin is responsible for a flight performance upona shot with a driver.

Other resin may be used in combination with the ionomer resin. In caseof the use in combination, the ionomer resin is included as a principalcomponent of the base polymer in light of the flight performance.Proportion of the ionomer resin occupying in the total base polymer ispreferably equal to or greater than 50% by weight, more preferably equalto or greater than 70% by weight, and particularly preferably equal toor greater than 85% by weight.

Preferably, an ionomer resin is used that is a copolymer of α-olefin andan α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms in which apart of the carboxylic acid is neutralized with a metal ion. Examples ofpreferable α-olefin include ethylene and propylene. Examples ofpreferable α,β-unsaturated carboxylic acid include acrylic acid andmethacrylic acid. Illustrative examples of the metal ion for use in theneutralization include sodium ion, potassium ion, lithium ion, zinc ion,calcium ion, magnesium ion, aluminum ion and neodymium ion. Theneutralization may also be carried out with two or more kinds of themetal ions. In light of the resilience performance and durability of thegolf ball 2, particularly suitable metal ions are sodium ion, zinc ion,lithium ion and magnesium ion.

Specific examples of the ionomer resin include trade names “Himilan1555”, “Himilan 1557”, “Himilan 1605”, “Himilan 1706”, “Himilan 1707”,“Himilan 1855”, “Himilan 1856”, “Himilan AM7311”, “Himilan AM7315”,“Himilan AM7317”, “Himilan AM7318” and “Himilan MK7320”, available fromDu Pont-MITSUI POLYCHEMICALS Co., Ltd.; trade names “Surlyn® 7930”,“Surlyn® 7940”, “Surlyn® 8140”, “Surlyn® 8940”, “Surlyn® 8945”, “Surlyn®9120”, “Surlyn® 9910” and “Surlyn® 9945”, available from Dupont; andtrade names “IOTEK 7010”, “IOTEK 7030”, “IOTEK 8000” and “IOTEK 8030”,available from EXXON Corporation.

Into the resin composition of the mid layer 6 may be blended a fillerfor the purpose of adjusting specific gravity and the like. Illustrativeexamples of suitable filler include zinc oxide, barium sulfate, calciumcarbonate and magnesium carbonate. Powder of a highly dense metal may bealso blended as a filler. Specific examples of the highly dense metalinclude tungsten and molybdenum. The amount of the filler to be blendedis determined ad libitum so that the intended specific gravity of themid layer 6 can be accomplished. Into the mid layer 6 may be alsoblended a coloring agent, crosslinked rubber powder or synthetic resinpowder.

The mid layer 6 has a hardness Hm of equal to or greater than 62. Thismid layer 6 is responsible for a flight performance upon a shot with adriver. In light of the flight performance, the hardness Hm of the midlayer 6 is preferably equal to or greater than 63. When the hardness Hmis extremely great, to achieve a favorable feeling upon impact of thegolf ball 2 may become difficult. In this respect, the hardness Hm ispreferably equal to or less than 72, more preferably equal to or lessthan 70, and particularly preferably equal to or less than 68.

In the present invention, the hardness Hm of the mid layer 6 and thehardness Hc of the cover 10 are measured in accordance with a standardof “ASTM-D 2240-68”. For the measurement, an automated rubber hardnessmachine which is equipped with a Shore D type spring hardness scale(trade name “LA1”, available from Koubunshi Keiki Co., Ltd.) is used.For the measurement, a sheet which is formed by hot press is used havinga thickness of about 2 mm and consisting of the same material as the midlayer 6 (or the cover 10). Prior to the measurement, the sheet is storedat a temperature of 23° C. for two weeks. When the measurement iscarried out, three sheets are overlaid.

It is preferred that the mid layer 6 has a thickness Tm of 0.3 mm orgreater and 2.5 mm or less. When the thickness Tm is less than the aboverange, a flight performance upon a shot with a driver may becomeinsufficient. In this respect, the thickness Tm is more preferably equalto or greater than 0.5 mm, still more preferably equal to or greaterthan 0.7 mm, and further preferably equal to or greater than 1.0 mm.When the thickness Tm is beyond the above range, to achieve a favorablefeeling upon impact of the golf ball 2 may become difficult. In thisrespect, the thickness Tm is more preferably equal to or less than 2.3mm.

In light of adhesion between the mid layer 6 and the reinforcing layer 8or the cover 10, the surface of the mid layer 6 is preferably subjectedto a surface treatment to increase the roughness thereof. Specificexamples of the treatment include brushing, grinding and the like.

In the reinforcing layer 8, a two-component cured thermosetting resinmay be suitably used as a base polymer. Specific examples of thetwo-component cured thermosetting resin include epoxy resins, urethaneresins, acrylic resins, polyester based resins and cellulose basedresins. In light of the mechanical strength (e.g., strength at break)and durability of the reinforcing layer 8, two-component cured epoxyresins and two-component cured urethane resins are preferred.

The two-component cured epoxy resin is obtained by curing an epoxy resinwith a polyamide based curing agent. Illustrative examples of the epoxyresin for use in the two-component cured epoxy resin include bisphenol Atype epoxy resin, bisphenol F type epoxy resin and bisphenol AD typeepoxy resin. The bisphenol A type epoxy resin is obtained by a reactionof bisphenol A with an epoxy group-containing compound such asepichlorohydrin. The bisphenol F type epoxy resin is obtained by areaction of bisphenol F with an epoxy group-containing compound. Thebisphenol AD type epoxy resin is obtained by a reaction of bisphenol ADwith an epoxy group-containing compound. In light of the balance amongsoftness, chemical resistance, heat resistance and toughness, thebisphenol A type epoxy resin is preferred.

The polyamide based curing agent has multiple amino groups and one ormore amide groups. This amino group can react with an epoxy group.Specific examples of the polyamide based curing agent include polyamideamine curing agents and denatured products of the same. The polyamideamine curing agent is obtained by a condensation reaction of apolymerized fatty acid with a polyamine. Typical polymerized fatty acidmay be obtained by heating natural occurring fatty acids containinglarge amounts of unsaturated fatty acids such as linoleic acid,linolenic acid and the like in the presence of a catalyst to perfect thesynthesis. Specific examples of the unsaturated fatty acid include talloil, soybean oil, linseed oil and fish oil. Polymerized fatty acidshaving a dimer content of equal to or greater than 90% by weight and atrimer content of equal to or less than 10% by weight, and beinghydrogenated are preferred. Illustrative examples of preferred polyamineinclude polyethylene diamine, polyoxyalkylene diamine and derivativesthereof.

Upon mixing of the epoxy resin and the polyamide based curing agent,ratio of epoxy equivalent of the epoxy resin and amine active hydrogenequivalent of the polyamide based curing agent is preferably 1.0/1.4 orgreater and 1.0/1.0 or less.

The two-component cured urethane resin is obtained by a reaction of abase material and a curing agent. A two-component cured urethane resinobtained by a reaction of a base material containing a polyol componentwith a curing agent containing polyisocyanate or a derivative thereof,or a two-component cured urethane resin obtained by a reaction of a basematerial containing isocyanate group-ended urethane prepolymer with acuring agent having an active hydrogen may be used. In particular,two-component cured urethane resins prepared by a reaction of a basematerial containing a polyol component with a curing agent containingpolyisocyanate or a derivative thereof are preferred.

It is preferred that urethane polyol is used as the polyol component ofthe base material. The urethane polyol has urethane bonds and at leasttwo hydroxyl groups. Preferably, the urethane polyol has a hydroxylgroup at its end. The urethane polyol may be obtained by allowing polyoland polyisocyanate to react at a ratio such that an excessive molarratio of the hydroxyl group of the polyol component to the isocyanategroup of polyisocyanate is attained.

The polyol for use in production of the urethane polyol has multiplehydroxyl groups. Polyol having a weight average molecular weight of 50or greater and 2000 or less, and particularly 100 or greater and 1000 orless is preferred. Examples of the polyol having a low molecular weightinclude diol and triol. Specific examples of the diol include ethyleneglycol, diethylene glycol, triethylene glycol, 1,3-butanediol,1,4-butanediol, neopentyl glycol and 1,6-hexanediol. Specific examplesof the triol include trimethylolpropane and hexanetriol. Examples of thepolyol having a high molecular weight include polyether polyols such aspolyoxyethylene glycol (PEG), polyoxypropylene glycol (PPG) andpolyoxytetramethylene glycol (PTMG); condensed polyester polyols such aspolyethylene adipate (PEA), polybutylene adipate (PBA) andpolyhexamethylene adipate (PHMA); lactone based polyester polyols suchas poly-ε-caprolactone (PCL); polycarbonate polyols such aspolyhexamethylene carbonate; and acrylic polyols. Two or more kinds ofpolyols may be used in combination.

Polyisocyanate for use in production of urethane polyol has multipleisocyanate groups. Specific examples of the polyisocyanate includearomatic polyisocyanates such as 2,4-toluene diisocyanate, 2,6-toluenediisocyanate, mixtures of 2,4-toluene diisocyanate and 2,6-toluenediisocyanate (TDI), 4,4′-diphenylmethane disocyanate (MDI),1,5-naphthylene diisocyanate (NDI), 3,3′-bitolylene-4,4′-diisocyanate(TODI), xylylene diisocyanate (XDI), tetramethylxylylene diisocyanate(TMXDI) and paraphenylene diisocyanate (PPDI); alicyclic polyisocyanatessuch as 4,4′-dicyclohexylmethane diisocyanate (H₁₂MDI), hydrogenatedxylylene diisocyanate (H₆XDI), hexamethylene diisocyanate (HDI) andisophorone diisocyanate (IPDI); and aliphatic polyisocyanates. Two ormore polyisocyanates may be used in combination. In light of the weatherresistance, TMXDI, XDI, HDI, H₆XDI, IPDI and H₁₂MDI are preferred.

In the reaction of polyol with polyisocyanate for producing the urethanepolyol, any known catalyst may be used. Typical catalyst may bedibutyltin dilaurate.

In light of strength of the reinforcing layer 8, ratio of the urethanebonds included in the urethane polyol is preferably equal to or greaterthan 0.1 mmol/g. In light of the following capability of the reinforcinglayer 8 to the cover 10, the ratio of the urethane bonds included in theurethane polyol is preferably equal to or less than 5 mmol/g. The ratioof the urethane bonds may be adjusted by adjusting the molecular weightof the polyol to be a raw material, and by adjusting compounding ratioof the polyol and the polyisocyanate.

In light of a short time period required for the reaction of the basematerial with the curing agent, the urethane polyol has a weight averagemolecular weight of preferably equal to or greater than 4000, and morepreferably equal to or greater than 4500. In light of the adhesivenessof the reinforcing layer 8, the urethane polyol has a weight averagemolecular weight of preferably equal to or less than 10000, and morepreferably equal to or less than 9000.

In light of the adhesiveness of the reinforcing layer 8, the urethanepolyol has a hydroxyl value (mgKOH/g) of preferably equal to or greaterthan 15, and more preferably equal to or greater than 73. In light of ashort time period required for the reaction of the base material withthe curing agent, the urethane polyol has a hydroxyl value of preferablyequal to or less than 130, and more preferably equal to or less than120.

The base material may contain, in addition to the urethane polyol, apolyol not having any urethane bond. The aforementioned polyol that is araw material of the urethane polyol may be used in the base material.Polyols that are miscible with the urethane polyol are preferred. Inlight of a short time period required for the reaction of the basematerial with the curing agent, proportion of the urethane polyol in thebase material is preferably equal to or greater than 50% by weight andmore preferably equal to or greater than 80% by weight based on thesolid content. Ideally, this proportion is 100% by weight.

The curing agent contains polyisocyanate or a derivative thereof. Theaforementioned polyisocyanate that is a raw material of the urethanepolyol may be used in the curing agent.

The reinforcing layer 8 may include additives such as a coloring agent(typically, titanium dioxide), a phosphate based stabilizer, anantioxidant, a light stabilizer, a fluorescent brightening agent, anultraviolet absorbent, a blocking preventive agent and the like. Theadditive may be added to the base material of the two-component curedthermosetting resin, or may be added to the curing agent.

The reinforcing layer 8 is obtained by coating a liquid, which isprepared by dissolving or dispersing a base material and a curing agentin a solvent, on the surface of the mid layer 6. In light of theworkability, coating with a spray gun is preferred. The solvent isvolatilized after the coating to permit a reaction of the base materialwith the curing agent thereby forming the reinforcing layer 8.Illustrative examples of preferred solvent include toluene, isopropylalcohol, xylene, methyl ethyl ketone, methyl isobutyl ketone, ethyleneglycol monomethyl ether, ethylbenzene, propylene glycol monomethylether, isobutyl alcohol and ethyl acetate.

The reinforcing layer 8 lies between the mid layer 6 and the cover 10 toelevate the adhesiveness therebetween. As described later, the cover 10of this golf ball 2 is very thin. When such a thin cover 10 is hit withan edge of a clubface, a wrinkle is liable to be generated. Thereinforcing layer 8 suppresses generation of such a wrinkle.

A clubface has a loft. Upon impact with this clubface, a torsion forceis applied to the golf ball. This force allows the cover 10 to deform ina circumferential direction. Presence of the reinforcing layer 8 leadsto suppression of such deformation. This reinforcing layer 8 achieves alow spin rate and a great launch angle, thereby attaining a great flightdistance upon a shot with a driver. Because of a synergistic effect ofthe mid layer 6 having a hardness Hm of equal to or greater than 62, thecover 10 having the ratio (Hc/Wc) of equal to or greater than 10.0 andthe reinforcing layer 8, an extremely excellent flight performance upona shot with a driver is accomplished. The reinforcing layer 8 alsosuppresses deterioration of the resilience performance resulting fromrepeated hits.

In light of suppression of a wrinkle and the flight performance upon ashot with a driver, the reinforcing layer 8 has a tensile strength ofpreferably equal to or greater than 150 kgf/cm², and more preferablyequal to or greater than 200 kgf/cm². Usually yielded tensile strengthis equal to or less than 500 kgf/cm². The tensile strength is measuredin accordance with a standard of “JIS K5400”. A sample subjected to themeasurement is obtained by applying a coating composition on a testplate with a spray gun. The coating composition is kept in an atmosphereof 40° C. for 24 hours. The strain rate for the measurement is 50mm/min.

In light of suppression of a wrinkle and the flight performance upon ashot with a driver, the reinforcing layer 8 has a pencil hardness ofpreferably equal to or greater than 4B, more preferably equal to orgreater than 3B, and particularly preferably equal to or greater than B.In light of the following capability to the cover 10 upon a hit of thegolf ball 2, the reinforcing layer 8 has a pencil hardness of preferablyequal to or less than 4H, more preferably equal to or less than 3H, andparticularly preferably equal to or less than 2H. The pencil hardness ismeasured in accordance with a standard of “JIS K5400”.

In light of suppression of a wrinkle, the reinforcing layer 8 has athickness of preferably equal to or greater than 3 μm, and morepreferably equal to or greater than 5 μm. In light of easy formation ofthe reinforcing layer 8, it is preferred that the thickness is equal toor less than 300 μm, still more, equal to or less than 100 μm, yet more,equal to or less than 50 μm, and further, equal to or less than 20 μm.The thickness is measured by observation of the cross section of thegolf ball 2 with a micro scope. When the surface of the mid layer 6 hasroughness resulting from the surface roughening treatment, the thicknessis measured immediately above the protruded portion.

When sufficient adhesion between the mid layer 6 and the cover 10 isaccomplished leading to less possibility to generate a wrinkle, thereinforcing layer 8 may not be provided.

A thermoplastic resin composition is suitably used for the cover 10.Examples of base polymer of this resin composition include thermoplasticpolyurethane elastomers, thermoplastic polyester elastomers,thermoplastic polyamide elastomers, thermoplastic polyolefin elastomers,thermoplastic polystyrene elastomers and ionomer resins. In particular,thermoplastic polyurethane elastomers are preferred. The thermoplasticpolyurethane elastomers are soft. Great spin rate is achieved upon a hitwith a short iron of the golf ball 2 having a cover 10 comprising athermoplastic polyurethane elastomer. The cover 10 comprising athermoplastic polyurethane elastomer is responsible for a controlperformance upon a shot with a short iron. The thermoplasticpolyurethane elastomer is also responsible for the scuff resistance ofthe cover 10.

Other resin may be used in combination with the thermoplasticpolyurethane elastomer. In light of the control performance, thethermoplastic polyurethane elastomer is included in the base polymer asa principal component when used in combination. Proportion of thethermoplastic polyurethane elastomer occupying in total base polymer ispreferably equal to or greater than 50% by weight, more preferably equalto or greater than 70% by weight, and particularly preferably equal toor greater than 85% by weight.

The thermoplastic polyurethane elastomer includes a polyurethanecomponent as a hard segment, and a polyester component or a polyethercomponent as a soft segment. Illustrative examples of the curing agentfor the polyurethane component include alicyclic diisocyanate, aromaticdiisocyanate and aliphatic diisocyanate. In particular, alicyclicdiisocyanate is preferred. Because the alicyclic diisocyanate has nodouble bond in the main chain, yellowing of the cover 10 can besuppressed. Additionally, because the alicyclic diisocyanate isexcellent in strength, the cover 10 can be prevented from being scuffed.Two or more kinds of diisocyanates may be used in combination.

Illustrative examples of the alicyclic diisocyanate include4,4′-dicyclohexylmethane diisocyanate (H₁₂MDI)1,3-bis(isocyanatomethyl)cyclohexane (H₆XDI), isophorone diisocyanate(IPDI) and trans-1,4-cyclohexane diisocyanate (CHDI). In light ofversatility and processability, H₁₂MDI is preferred.

Illustrative examples of the aromatic diisocyanate include4,4′-diphenylmethane diisocyanate (MDI) and toluene diisocyanate (TDI).Illustrative examples of the aliphatic diisocyanate includehexamethylene diisocyanate (HDI).

Specific examples of the thermoplastic polyurethane elastomer includetrade name “Elastollan XNY90A”, trade name “Elastollan XNY97A”, tradename “Elastollan XNY585” and trade name “Elastollan XKPO16N”, availablefrom BASF Japan Ltd; and trade name “Rezamin P4585LS” and trade name“Rezamin PS62490”, available from Dainichiseika Color & Chemicals Mfg.Co., Ltd.

Into the cover 10 may be blended a coloring agent such as titaniumdioxide, a filler such as barium sulfate, a dispersant, an antioxidant,an ultraviolet absorbent, a light stabilizer, a fluorescent agent, afluorescent brightening agent and the like in an appropriate amount asneeded. Also, the cover 10 may be blended with powder of a highly densemetal such as tungsten, molybdenum or the like for the purpose ofadjusting the specific gravity.

The cover 10 has a Shore D hardness Hc of equal to or less than 55. Byemploying such a soft cover 10, a favorable control performance may beaccomplished upon a shot with a short iron. In light of the controlperformance, the hardness Hc is more preferably equal to or less than50, and particularly preferably equal to or less than 47. When thehardness is too small, a flight performance upon a shot with a drivermay be insufficient. In this respect, it is preferred that the hardnessHc is equal to or greater than 20, still more, equal to or greater than28, and further, equal to or greater than 33.

Ratio (Hc/Wc) of the hardness Hc to the weight Wc (g) of the cover 10 isequal to or greater than 10.0. The ratio (Hc/Wc) is a marker thatrepresents the extent of contribution of the cover 10 to the flightperformance upon a shot with a driver. By setting the ratio (Hc/Wc) tobe equal to or greater than 10.0, a great flight distance is attainedirrespective of the softness of the cover 10. In light of the flightperformance, the ratio (Hc/Wc) is preferably equal to or greater than15.0, and more preferably equal to or greater than 20.0. In light of thecontrol performance upon a shot with a short iron, the ratio (Hc/Wc) ispreferably equal to or less than 100, and more preferably equal to orless than 50.

The cover 10 has a weight Wc of equal to or less than 3.2 g. Asdescribed above, the cover 10 has a low hardness. The cover 10 havingsuch a low hardness is disadvantageous in terms of a resiliencecoefficient of the golf ball 2. Upon a shot with a driver, the mid layer6 as well as the core 4 of the golf ball 2 is deformed greatly. Bysetting the weight Wc to be equal to or less than 3.2 g, the cover 10does not adversely affect the resilience coefficient to a large extentupon a shot with a driver, even though the cover 10 has a low hardness.An excellent flight performance can be accomplished upon a shot with adriver through the combination of this cover 10 and the mid layer 6having a high hardness.

In light of the flight performance, the weight Wc is preferably equal toor less than 2.6 g, and more preferably equal to or less than 2.3 g.When the weight Wc is too small, a difficulty may be involved in formingthe cover 10. In this respect, the weight Wc is preferably equal to orgreater than 0.6 g, and more preferably equal to or greater than 1.0 g.

In light of the flight performance upon a shot with a driver, thethickness Tc of the cover 10 is preferably equal to or less than 0.6 mm,more preferably equal to or less than 0.5 mm, and particularlypreferably equal to or less than 0.4 mm. In light of ease in forming,the thickness Tc is preferably equal to or greater than 0.1 mm, and morepreferably equal to or greater than 0.2 mm.

EXAMPLES Example 1

A rubber composition was obtained by kneading 100 parts by weight ofpolybutadiene (trade name “BR-730”, available from JSR Corporation), 35parts by weight of zinc diacrylate, an appropriate amount of zinc oxide,0.7 part by weight of bis(pentabromophenyl)disulfide and 0.9 part byweight of dicumyl peroxide. This rubber composition was placed into amold having upper and lower mold half each having a hemisphericalcavity, and heated under a temperature of 170° C. for 15 minutes toobtain a core having a diameter of 38.5 mm. This core had a weight of34.9 g.

A resin composition was obtained by kneading 50 parts by weight of anionomer resin (Himilan 1605, described above) 50 parts by weight ofother ionomer resin (Surlyn® 9945, described above), 4 parts by weightof titanium dioxide and 0.1 part by weight of a coloring agent(ultramarine blue) in a biaxial extruder. This resin composition wasrendered to cover around the core by injection molding to obtain a midlayer. This mid layer had a hardness Hm of 63.

A coating composition containing a two-component cured epoxy resin as abase polymer (trade name “POLIN 750LE”, available from Shinto Paint Co.,Ltd.) was prepared. The base material liquid of this coating compositionconsists of 30 parts by weight of bisphenol A type solid epoxy resin and70 parts by weight of a solvent. The curing agent liquid of this coatingcomposition consists of 40 parts by weight of denatured polyamide amine,55 parts by weight of a solvent and 5 parts by weight of titaniumdioxide. Weight ratio of the base material liquid and the curing agentliquid is 1/1. This coating composition was applied on the surface ofthe mid layer with a spray gun, and kept in an atmosphere of 40° C. for24 hours to give a reinforcing layer. The reinforcing layer had athickness of 10 μm.

A type c resin composition shown in Table 1 below was obtained with abiaxial extruder. Half shells were obtained from this resin compositionwith compression molding. A spherical body comprising the core, the midlayer and the reinforcing layer were covered by two pieces of the halfshell, which was placed into a mold having upper and lower mold halfeach having a hemispherical cavity to obtain a cover with compressionmolding. The cover had a weight of 2.6 g. A paint layer was formedaround this cover to give a golf ball of Example 1. TABLE 1Specification of cover (parts by weight) Type a b c d e Rezamin PS62490100 — — — — Rezamin P4585LS — 100 — — — Elastollan XNY90A — — 100 — —Elastollan XNY97A — — — 50 — Elastollan XKP016N — — — 50 100 Titaniumdioxide 4 4 4 4 4 Ultramarine blue 0.1 0.1 0.1 0.1 0.1 Hardness (ShoreD) 28 33 42 54 58

Examples 2 and 3

In a similar manner to Example 1 except that the thickness and the typeof the cover were as presented in Table 2 below, golf balls of Examples2 and 3 were obtained.

Examples 4 to 6 and Comparative Example 1

In a similar manner to Example 1 except that the type of the cover wasas presented in Table 2 below, golf balls of Examples 4 to 6 andComparative Example 1 were obtained.

Example 7

In a similar manner to Example 1 except that the thickness of the midlayer and the thickness of the cover were as presented in Table 3 below,golf ball of Example 7 was obtained.

Comparative Example 2

In a similar manner to Example 1 except that the thickness of the midlayer, and the thickness and the type of the cover were as presented inTable 3 below, golf ball of Comparative Example 2 was obtained.

Examples 8 to 10

In a similar manner to Example 1 except that the thickness of thereinforcing layer was as presented in Table 3 below, golf balls ofExamples 8 and 9 were obtained. In a similar manner to Example 1 exceptthat no reinforcing layer was provided, golf ball of Example 10 wasobtained.

Example 11

In a similar manner to Example 1 except that a coating compositioncomprising a two-component cured urethane resin as a base polymer wasused in the reinforcing layer, golf ball of Example 11 was obtained. Inproduction of this coating composition, 116 parts by weight of PTMG and16 parts by weight of 1,2,6-hexanetriol were first dissolved in 120parts by weight of a solvent (mixed liquid of toluene and methyl ethylketone). To this solution was added dibutyltin dilaurate to give theconcentration of 0.1% by weight. To this solution was added 48 parts byweight of isophorone diisocyanate dropwise while keeping at 80° C. toobtain a base material liquid containing urethane polyol. Solid contentof this urethane polyol was 60% by weight, with a hydroxyl value being87 mgKOH/g, and with a weight average molecular weight being 7850. Thisbase material liquid and a curing agent liquid containing isophoronediisocyanate (manufactured by Sumitomo Bayer Urethane Co., Ltd.) weremixed to give a molar ratio of NCO/OH being 1.2. To this liquid wereadded a light stabilizer (trade name “Sanol LS770”, available fromSankyo Co., Ltd.), an ultraviolet ray absorbing agent (trade name“TINUVIN® 900”, available from Ciba-Geigy Co.) and a fluorescentbrightening agent (trade name “UVITEX® OB”, available from Ciba-GeigyCo.”) to prepare a coating composition. Amounts as added per 100 partsby weight of the urethane resin component are 2 parts by weight for thelight stabilizer, 2 parts by weight for the ultraviolet ray absorbingagent and 0.2 part by weight for the fluorescent brightening agent.

Comparative Example 3

In a similar manner to Example 1 except that a resin compositioncomprising 50 parts by weight of an ionomer resin (Himilan 1855,described above), 50 parts by weight of other ionomer resin (Himilan1856, described above), 4 parts by weight of titanium dioxide and 0.1part by weight of a coloring agent (ultramarine blue) was used in themid layer, golf ball of Comparative Example 3 was obtained.

[Shot with Driver]

A driver with a metal head was attached to a swing machine availablefrom Golf Laboratory Co. Then the machine condition was set to give thehead speed of 50 m/sec, and the golf balls were hit therewith.Accordingly, ball speed immediately after the hit and travel distance(i.e., the distance from the launching point to the point where the ballstopped) were measured. Mean values of 10 times measurement are shown inTable 2 and Table 3 below.

[Shot with Short Iron]

To the swing machine described above was attached an approach wedge.Then the machine condition was set to give the head speed of 21 m/sec,and the golf balls were hit therewith. Accordingly, spin rateimmediately after the hit was measured. Mean values of 10 timesmeasurement are shown in Table 2 and Table 3 below.

[Evaluation of Extent of Wrinkle Generation]

To the swing machine described above was attached a pitching wedge.Machine height was adjusted such that the golf ball is hit at a leadingedge of the club head. Then the machine condition was set to give thehead speed of 37 m/sec, and the golf balls were hit therewith.Accordingly, the surface of the golf ball was visually observed, and theextent of a wrinkle was rated into four ranks of from A to D based onthe following criteria:

A: wrinkle hardly generated;

B: wrinkle slightly generated;

C: wrinkle greatly generated; and

D: wrinkle greatly generated, with exposed mid layer.

The results are presented in Table 2 and Table 3 below.

[Evaluation of Durability]

The golf balls were rendered to hit on a metal plate repeatedly at avelocity of 45 m/s. Durability was rated into four ranks of from A to Dbased on the following criteria:

A: no breakage caused on 150 times hitting;

B: no breakage caused on 100 times hitting, although breakage caused on150 times hitting;

C: no breakage caused on 50 times hitting, although breakage caused on100 times hitting; and

D: breakage caused on 50 times hitting.

The measurement was carried out on 6 golf balls. The results of ratingwhich gave a maximum convergence are presented in Table 2 and Table 3below. TABLE 2 Results of evaluation Compara. Example Example ExampleExample Example Example example 2 3 4 5 1 6 1 Mid layer Hardness Hm 6363 63 63 63 63 63 Thickness Tm (mm) 2.0 2.0 1.6 1.6 1.6 1.6 1.6Reinforcing layer Base polymer epoxy epoxy epoxy epoxy epoxy epoxy epoxyThickness (μm) 10 10 10 10 10 10 10 Ttensile strength (kgf/cm²) 410 410410 410 410 410 410 Pencil Hardness 2H 2H 2H 2H 2H 2H 2H Cover Weight Wc(g) 0.6 0.6 2.6 2.6 2.6 2.6 2.6 Thickness Tc (mm) 0.1 0.1 0.5 0.5 0.50.5 0.5 Type a b a b c d e Hardness Hc 28 33 28 33 42 54 58 Hc/Wc 46.755.0 10.8 12.7 16.2 20.8 22.3 Shot with driver Ball speed (m/s) 72.573.1 72.4 72.6 72.7 72.8 72.9 Travel distance (m) 272.5 277.1 272.2273.2 275.2 276.4 277.1 Spin rate upon shot with approach wedge (rpm)7200 6800 7300 7100 6700 6400 5900 Extent of wrinkle C B A A A A ADurability A A A A A A A

TABLE 3 Results of evaluation Compara. Compara. Example example ExampleExample Example Example example 7 2 8 9 10 11 3 Mid layer Hardness Hm 6363 63 63 63 63 55 Thickness Tm (mm) 1.5 1.4 1.6 1.6 1.6 1.6 1.6Reinforcing layer Base polymer epoxy epoxy epoxy epoxy — polyurethaneepoxy Thickness (μm) 10 10 50 5 — 10 10 Ttensile strength (kgf/cm²) 410410 410 410 — 221 410 Pencil Hardness 2H 2H 2H 2H — B 2H Cover Weight Wc(g) 3.2 3.8 2.6 2.6 2.6 2.6 2.6 Thickness Tc (mm) 0.6 0.7 0.5 0.5 0.50.5 0.5 Type c b c c c c c Hardness Hc 42 33 42 42 42 42 42 Hc/Wc 16.98.7 16.2 16.2 16.2 16.2 16.2 Shot with driver Ball speed (m/s) 72.6 72.072.8 72.7 72.7 72.6 72.1 Travel distance (m) 273.0 266.1 276.8 275.1275.2 272.9 267.0 Spin rate upon shot with approach wedge (rpm) 67507400 6700 6700 6700 6700 6750 Extent of wrinkle A A A B D B A DurabilityA A A A D A A

As is clear from Table 2 and Table 3, the golf ball of each of Examplesis excellent in the flight performance upon shots with a driver, and inthe spin performance upon shots with a short iron. Accordingly,advantages of the present invention are clearly indicated by theseresults of evaluation.

The description herein above is merely for illustrative examples, andvarious modifications can be made without departing from the principlesof the present invention.

1. A golf ball which comprises a spherical core, a mid layer positionedoutside of the core, and a cover positioned outside of the mid layer,principal component of the base polymer of said mid layer being anionomer resin, said mid layer having a hardness Hm as measured with aShore D type hardness scale of equal to or greater than 62, said coverhaving a hardness Hc as measured with a Shore D type hardness scale ofequal to or less than 55, said cover having a weight Wc of equal to orless than 3.2 g, and ratio (Hc/Wc) of the hardness Hc to the weight Wc(g) of said cover being equal to or greater than 10.0.
 2. The golf ballaccording to claim 1 wherein a principal component of the base polymerof said cover is a thermoplastic polyurethane elastomer.
 3. The golfball according to claim 1 wherein said cover has a thickness Tc of equalto or less than 0.6 mm.
 4. The golf ball according to claim 1 wherein areinforcing layer which comprises a thermosetting resin as a basepolymer is provided between said mid layer and said cover.
 5. The golfball according to claim 4 wherein said reinforcing layer has a thicknessof 3 μm or greater and 50 μm or less.
 6. The golf ball according toclaim 4 wherein said reinforcing layer has a tensile strength of 150kgf/cm² or greater and 500 kgf/cm² or less.